Treat the patient in front of you. If they are unwell, assess ABCDE and call 999 first. The ECG supports a clinical decision and does not replace one. Recording or sending an ECG must never delay emergency transfer (NICE CG95).
The Resuscitation Council UK tachycardia and bradycardia algorithms turn on the same four adverse features: shock, syncope, myocardial ischaemia and heart failure. Any of them means the patient is unstable and needs emergency treatment, whatever the exact rhythm. In primary care that means 999, not a referral letter.
A normal-looking ECG in an unwell patient is not reassuring. Ischaemia can evolve, paroxysmal arrhythmias are often absent between episodes, and severe hyperkalaemia can be present with a normal trace.
NICE CG109 (transient loss of consciousness) applies to adults and young people aged 16 and over. Paediatric ECG reference values differ by age, so adult cut-offs produce both false alarms and false reassurance in children.
Most misreads start before the first wave: the wrong patient, the wrong calibration, a reversed lead or no old ECG for comparison.
Automated algorithms miss subtle ST elevation, pre-excitation, Brugada and epsilon waves, and they over-call artefact. Their QTc measurement is particularly unreliable in AF, bundle branch block and with flat or notched T waves. CG109 expects the ECG in TLoC to be interpreted by a healthcare professional trained and competent to identify the red-flag abnormalities.
Every ECG needs a named clinician who reads it, signs it and decides what happens next. An abnormal ECG left unfiled in an inbox is a recognised cause of harm.
Read in the same order every time, using the rhythm strip (usually lead II) for rate and rhythm. Adult values.
Heart rate alone rarely decides urgency: a rate of 45 in a marathon runner can be normal, while 45 from complete heart block in an older patient with dizziness is an emergency. Rhythm, the P-QRS relationship, symptoms and haemodynamics decide.
Atrial flutter with 2:1 block produces a regular narrow-complex tachycardia at about 150 /min that is easily misread as sinus tachycardia or SVT. Every second flutter wave is often hidden in the QRS or T wave.
Fascicular block criteria follow the AHA/ACCF/HRS recommendations for the standardisation and interpretation of the ECG (2009).
This is where the dangerous findings live: broad QRS, ST elevation, new T-wave inversion and a long QT.
The Fourth Universal Definition of Myocardial Infarction (2018) sets the J-point thresholds above. They are a guide, not a rule-out: a STEMI-equivalent (for example posterior MI, or ST changes in LBBB) may not meet them, and a normal ECG does not exclude ACS (CG95).
QTc depends on the correction formula, heart rate, ECG quality, sex and age, so a single automated QTc just over a cut-off is a reason to measure it yourself and repeat it, not a diagnosis of long QT syndrome. Rising risk with QTc ≥500 ms, or a rise of over 60 ms from baseline, is the commonly used threshold for stopping a QT-prolonging drug (Trinkley et al., 2013).
Common QT-prolonging medicines include macrolides and fluoroquinolones, many antipsychotics, citalopram and escitalopram, methadone, ondansetron, domperidone, hydroxychloroquine and the class III antiarrhythmics. Check the current SPC and local formulary, and remember additive risk from combinations, interacting drugs, low potassium or magnesium, bradycardia, female sex, older age and structural heart disease.
Urgency comes from the finding together with symptoms, haemodynamics and whether it is new. If in doubt, go up a tier.
Any of these with syncope or presyncope, exertional symptoms, a family history of sudden cardiac death under 40, heart failure, new breathlessness or a murmur moves up the ladder — CG109's 24-hour cardiovascular assessment applies to TLoC with any of those features.
NICE CG109 asks for specialist cardiovascular assessment within 24 hours for anyone with TLoC who has an ECG abnormality, heart failure, TLoC during exertion, a family history of sudden cardiac death under 40 or an inherited cardiac condition, new or unexplained breathlessness, or a heart murmur; consider the same for people over 65 with TLoC without prodromal symptoms. The ECG list is NICE's list for TLoC; outside TLoC, use it as a guide to which findings matter.
The Resuscitation Council UK bradycardia algorithm names recent asystole, Mobitz II, complete heart block with broad QRS and ventricular pauses over 3 s as risks of asystole. These patients can deteriorate suddenly even when they look well, which is why they do not wait for a routine referral.
Chest-pain tiers follow NICE CG95: current pain is an emergency; pain in the last 12 hours with an abnormal or unavailable ECG is an emergency referral; pain-free with a normal ECG, or pain 12–72 hours ago, needs urgent same-day assessment.
What you will see most often in general practice, with what to do about it. Adverse features or ongoing symptoms always override the action shown.
Related pathways: see Palpitations · Transient loss of consciousness · Dizziness · Atrial fibrillation
NICE NG196 asks for a 12-lead ECG to confirm AF or flutter in anyone with an irregular pulse, with or without symptoms, and for CHA₂DS₂-VASc (stroke) and ORBIT (bleeding) assessment, including in atrial flutter. NICE allows emergency electrical cardioversion without waiting for anticoagulation in life-threatening haemodynamic instability from new-onset AF, which is why fast AF with adverse features is a 999 call.
Isolated complete RBBB is a common finding in healthy people. LBBB is less often benign and makes the ECG unreliable for ischaemia, which is why a presumed new LBBB with symptoms suggesting ACS is handled like ST elevation (CG95).
These are the patterns where a missed finding causes the most harm, often in a young or well-looking patient.
Related pathways: see Chest pain · Breathlessness · Hyperkalaemia · Hypokalaemia · Stable angina · Hypertension
Pre-excited AF is the reason AV-nodal blocking drugs are dangerous in WPW: with the node blocked, conduction goes down the accessory pathway and can degenerate into VF.
Brugada syndrome, congenital long QT, HCM and ARVC are inherited causes of sudden cardiac death in young people with often normal examinations. The ECG may be the only clue, and the family history of sudden death under 40 is the question to ask every time.
Hyperkalaemia ECG changes are a late and insensitive sign. The site's hyperkalaemia pathway (UK Kidney Association 2023) treats any attributable ECG change as an emergency whatever the potassium value.
Only once Steps 1 and 5 have been answered. Treat the cause, not the trace, and set a review date.
Next steps: see Atrial fibrillation · DOACs, antiplatelets and anticoagulation · Heart failure · Blackouts · Driving and the DVLA
NICE NG196 offers a standard beta-blocker (not sotalol) or a rate-limiting calcium-channel blocker as initial rate control, and advises against long-term amiodarone for rate control. Anticoagulation decisions should not wait for a specialist appointment.
Drug-induced QT prolongation is usually multifactorial: a second QT drug, an interacting medicine that raises levels, diuretic-induced hypokalaemia or a new bradycardia. Correcting the reversible factors is often enough, but only after the danger has been excluded.
The ECG is one snapshot. Most ECG-related harm comes from false reassurance, delay or an abnormal trace that nobody acted on.
Related pathways: see Palpitations · Transient loss of consciousness · Chest pain · Breathlessness · Dizziness · Hyperkalaemia
NICE CG95 is explicit that a normal resting 12-lead ECG does not exclude ACS, and that if the diagnosis is in doubt you should consider serial ECGs, review previous ECGs and record additional leads.
Many patients with an inherited arrhythmia syndrome have had an abnormal ECG recorded before the event. Writing down an interpretation, comparing with previous traces and acting on the abnormal ones is the safety net.